My Life on Mountain Railroads by John Gilbert Gould is a first hand account of working on the D&RG, Rio Grande Western and the Utah Railway. It often gets passed over by railfans but is a great book and is available for free online and as far as I know is still in print.
Utahrails.com mostly covers Utah but also has a large section on Colorado roads like the South Park and the Colorado Central
You also can't go wrong with anything written by Dan Abbot
Lanthrop's books are less history and more folklore, I would personally avoid them if you are trying to learn real history
Because I know others will suggest them here, I would avoid anything by Beebe & Clegg
Beebe & Clegg were known to blatantly lie and make up facts to spin a narrative. They had little understanding of actual railroading and filled their books with loaded sentences. They also made it pretty clear they had a genuine distaste for the working class and Native Americans, they basically open one of their books calling Native Americans irrelevant target practice and the pioneers,prospectors and farmers as simple hicks.